Aqueous polymer systems such as polymer dispersions and solutions for inks, coatings, and adhesives have been known for long and usually provides excellent technical properties and high occupational safety. The polymer may be constructed from plenty of polymer building blocks, such as alcohols, acids, isocyanates, vinyl compounds and others. Many of these polymer dispersions are typically more soluble in organic solvents than in water. Due to environmental concerns, e.g. disposal of solvents, safety issues and toxicity, development of aqueous polymer dispersions have been explored during the last decade. One approach to make an aqueous polymer dispersion is to incorporate ionizable building blocks into the polymer.
However, a key component for formation of anionically stabilised polymer dispersions is a reactive anionic building block. Considering practical aspects such as availability and technical performance, the selection of products is more or less reduced to 2,2-bis(hydroxymethyl)propanoic acid, DMPA, which is a diol monocarboxylic acid, or 5-Sulfo-1,3-benzenedicarboxylic acid, SIPA. The diol functionality of DMPA provides possibility of incorporation into polymers. E.g. by reacting the diol functionality with a carboxylic acid or isocyanate to form ester or carbamate bonds, respectively.
However, DMPA has some disadvantages such as high melting point and low solubility in various solvents, that makes it hard to handle in a process. To be able to dissolve DMPA awkward solvents are used, such as N-methylpyrrolidone or acetone. Furthermore, the ionic character is provided by a regular carboxylic group. It is not activated by e.g. inductive effects, which means that it is of low acidity and has a high protonation constant, pKa, which necessitate high pH of aqueous dispersion to ionize it. Furthermore, SIPA is very difficult to use as a resin building block due to low reactivity and sulphonate based dispersion are poorly buffered and suffers from uncontrollable pH fluctuation.
GB 1,455,554 discloses anionic building blocks, e.g. diols containing sulphuric acid or sulphonate groups. These diols would, if incorporated into polymers, have favourable effects. However, there are not many existing sulphonated products available on the market, mainly due to difficulties of incorporating them into preferred systems. A problem with diols containing sulphonate groups is a need to add a buffer, when incorporated into polymer systems, to provide pH stability.
When applying a coating system of aqueous dispersion type, such as a polyurethane dispersion, based on DMPA, on a wood substrate, the pH of the top surface wood layer will shift into an unnatural region.
Wood is a very complex material composed of e.g., polycarbohydrates such as cellulose, poly(phenylpropene) structures such as lignin, resins, oils, and fatty acids. A shift of pH from natural into unnatural will affect the structures of wood giving it less desirable properties. Most notably, pH sensitive chromophores will shift colour conferring the coated wood substrate an unnatural, unattractive appearance.
EP 605,858 and U.S. Pat. No. 5,470,899 disclose a process for reducing the pH of anionic polyurethane dispersions, which are stabilized by carboxylate or sulphonate groups, by addition of compounds which have one or more ester groups.
WO 9,806,768 discloses water-based polymers and a method for the preparation of water-based sulphonated polymer composition having a pH stability of greater than 2.0. Although several systems claims to be free of organic solvents there is a need for minor amounts of organic solvents in the preparation of a prepolymer.
Many water dispersible polyesters, such as alkyds, face difficulties because the ester bonds of the polymers are subjected to degradation due to hydrolysis. The shelf life of water dispersible polymer dispersions used in the ink, coating, and adhesive industry is dependent, to a large extent, of the integrity of the ester linkage within the polymer dispersions. Cleavage of the ester bonds in polymer dispersions during storage lowers the molecular weight of the polymers and impairs the performance of the ink, coating or adhesive containing the polymer dispersion. Therefore, methods for improving the hydrolytic stability of polymer dispersions are desirable.
US2004/0152830 claims increased stability of aqueous dispersions by inclusion of hydrolytically stable ester linkages formed from secondary or tertiary hydroxyl groups. This finding is of very limited help to the polyester designer, since most of the available hydroxy building blocks are primary alcohols. The aqueous stability of esters is highly dependent on pH. The low pH of aqueous systems, made possible by the present invention, is close to optimal for restraining ester hydrolysis. Furthermore, secondary and tertiary alcohols are both of low reactivity and of low thermal and acid stability in comparison with primary alcohols.
The selection of solvents for production of polyurethane dispersions is largely limited to N-methylpyrrolidone and a few other solvents. The restrictions are imposed due, amongst other thing, to the high hydrolytic activity of high pH aqueous solutions. Ester solvents and other hydrolytically sensitive additives are normally excluded using contemporary technique.
U.S. Pat. No. 6,576,702 discloses a polyurethane dispersion where a plasticizer, such as esters, is added to the prepolymer before it is dispersed in water.
From the above mentioned references it becomes clear that there is a need for building blocks for polymer systems, e g aqueous polymer systems, that can take care of the disadvantages mentioned above, i.e. provide aqueous dispersability, stability at low pH, retention of natural wood colour, and adequate reactivity.
Therefore, it is beneficial to develop a concept for waterborne polymers with high aqueous stability, great freedom in selection of raw materials, and minimal influence on the colour of wood substrates.